1. Field of the Invention
The present invention generally relates to a gallium nitride-based light emitting device and a method for manufacturing the same, and, more particularly, to a gallium nitride-based light emitting device, designed to have enhanced tolerance to reverse electrostatic discharge (ESD), and a method for manufacturing the same.
2. Description of the Related Art
Generally, a conventional gallium nitride-based light emitting device comprises a buffer layer, an n-type GaN-based clad layer, an active layer, and a p-type GaN-based clad layer sequentially stacked on a dielectric sapphire substrate in this order. Additionally, a transparent electrode and a p-side electrode are sequentially formed on the p-type GaN-based clad layer, and an n-side electrode is formed on a portion of the n-type GaN-based clad layer exposed by mesa etching. In such a conventional gallium nitride-based light emitting device, holes from the p-side electrode and electrons from the n-side electrode are coupled to emit light corresponding to energy band gap of a composition of the active layer.
Although the gallium nitride-based light emitting device has a significantly large energy band gap, it is vulnerable to electrostatic discharge due to its negative crystallinity. In particular, as the amount of crystal defects is increased, the light emitting device is more vulnerable to the electrostatic discharge. Specifically, the gallium nitride-based light emitting device based on a material having the formula AlxGayIn1-x-yN (0≦x≦1, 0≦y≦1) has a tolerance voltage of about 1 to 3 kV against forward ESD, and a tolerance voltage of about 100 V to 1 kV against reverse ESD. As such, the gallium nitride-based light emitting device is more vulnerable to the reverse ESD rather than the forward ESD. Thus, when a large reverse ESD voltage is applied in a pulse shape to the gallium nitride-based light emitting device, the light emitting device is deteriorated or damaged. For example, when the light emitting device is brought into contact with a person's body, or inserted into or drawn from a socket, a reverse ESD voltage of 10 kV or more is applied to the gallium nitride-based light emitting device. As a result, such a reverse ESD phenomenon damages reliability of the gallium nitride-based light emitting device as well as causing a sharp reduction in life span thereof.
In order to solve the above mentioned problem, several approaches for enhancing the tolerance voltage of the gallium nitride-based light emitting device against ESD have been proposed. For example, there is a method of enhancing the tolerance voltage of the light emitting device to ESD by optimizing the structure of the light emitting device, and process of manufacturing the same. However, with this method, there is a limitation in achieving desired tolerance to ESD. As another method, a light emitting diode (which will be referred to hereinafter as “LED”) of flip-chip structure is connected in parallel to a Si-based Zener diode so as to protect the light emitting device from ESD. However, in this method, an additional Zener diode must be purchased, and then assembled thereto by bonding, thereby significantly increasing material costs and manufacturing costs as well as restricting miniaturization of the device. As yet another method, U.S. Pat. No. 6,593,597 discloses technology for protecting the light emitting device from ESD by integrating an LED and a Schottky diode on an identical substrate and connecting them in parallel.
FIG. 1a is a cross-sectional view illustrating a conventional gallium nitride-based light emitting device having a Schottky diode connected in parallel as described above, and FIG. 1b is an equivalent circuit diagram of FIG. 1a. Referring to FIG. 1a, an LED structure of the conventional light emitting device comprises a first nucleus generation layer 102a, a first conductive buffer layer 104a, a lower confinement layer 106, an active layer 108, an upper confinement layer 110, a contact layer 112, a transparent electrode 114, and an n-side electrode 116 on a transparent substrate 100. Independent of the LED structure, a Schottky diode of the light emitting device comprises a second nucleus generation layer 102b and a second conductive buffer layer 104b formed on the transparent substrate 100, and a Schottky contact electrode 118 and an ohmic contact electrode 120 formed on the second conductive buffer layer 104b. 
The transparent electrode 114 of the LED structure is connected to the ohmic contact electrode 120, and the n-side electrode 116 of the LED structure is connected to the Schottky contact electrode 118. As a result, as shown in FIG. 1b, the light emitting device has a structure wherein the LED is connected to the Schottky diode in parallel. In the light emitting device constructed as described above, when a high reverse voltage, for example, a reverse ESD voltage is instantaneously applied thereto, the high voltage can be discharged through the Schottky diode. Accordingly, most currents flow through the Schottky diode rather than the LED, thereby reducing damage of the light emitting device.
However, the method of protecting the light emitting device from ESD using the Schottky diode has a drawback of complicated manufacturing process. In other words, not only a region for LED must be divided from a region for the Schottky diode, but also it is necessary to deposit an additional electrode material in ohmic contact with an electrode material constituting the Schottky diode on the second conductive buffer layer 104b comprising n-type GaN-based materials. In particular, there are problems of limitation in selection of the metallic material forming Schottky contact between the n-type GaN-based materials, and of possibility of change in contact properties of semiconductor-metal in following processes, such as heat treatment.